1. Field of the Invention
The present invention relates to the vertical stacking of conventional integrated circuit packages to increase the density of components on a printed circuit board. More particularly, the present invention relates to the vertical stacking of conventional memory integrated circuits packages on a surface mount printed circuit board.
2. Description of the Related Art
An integrated circuit or "IC" is a microcircuit formed from active and passive electrical components interconnected on or within a single semiconductor substrate. To protect the IC and to facilitate connection of the IC to a printed circuit board, off-the-shelf ICs are commonly packaged within a ceramic, plastic or epoxy IC package having multiple external terminals or "pins." The full integrated circuit package, including the IC, is commonly referred to (and will be referred to herein) as a "chip."
As a result of the continuously increasing demand for large random access computer memories, and the demand for smaller computers, various techniques have been developed to increase densities of memory chips on printed circuit boards. In addition to the inherent size advantages provided, increased chip densities enable shorter circuit paths between components, allowing the components to operate at higher speeds while reducing radio-frequency interference (RFI) and electromagnetic interference (EMI) emitted from the printed circuit board.
One development that has led to a significant increase in memory chip densities has been the advent of surface mount technology. With surface mount technology, conventional plated through holes on printed circuit boards are replaced with conductive pads, and through-hole pins of conventional chips are replaced with smaller surface mount pins. Because the pitch or spacing between centers of adjacent surface mount pins is significantly less than the conventional 0.10-inch spacing for conventional through-hole components, surface mount chips tend to be considerably smaller than corresponding conventional chips, and thus take up less space on the printed circuit board. Surface mount technology additionally facilitates the mounting of components on both sides of the printed circuit board.
Various techniques have been developed for increasing chip densities on printed circuit boards by vertically stacking or "piggybacking" two or more chips. See, for example, U.S. Pat. No. 4,996,583 to Hatada, U.S. Pat. No. 4,398,235 to Lutz et al., U.S. Pat. No. 4,953,005 to Carlson et al., Japanese Patent Publication No. 61-63048 to Toshiba Corp., Japanese Patent Publication No. 58-219757 to Tokyo Shibaura Denki K. K., Japanese Patent Publication No. 61-75558 to NEC Corp., and Japanese Patent Publication No. 60-254762 to Fujitsu. These techniques, however, tend to suffer from a number of defects. For instance, many of the techniques require the manufacture of custom chips that are specifically designed for stacking, or else require special modifications to the pins of standard memory chips. Further, many of the techniques do not make use of the various advantages of surface mount technology, such as the ability to maintain a low-profile when memory chips are mounted to the printed circuit board. Further, many proposed techniques for stacking memory chips are not cost effective.